Arcing faults cause many fires in homes, businesses, and industrial plants. Such faults often go undetected by conventional fault detectors that respond only to abnormally high levels of current in a circuit. For example, circuit breakers do not trip unless an amount of current above their rated value is sustained in their circuit.
In contrast to other types of faults (e.g., bolted faults) which can be detected by conventional circuit breakers, arcing faults occur periodically for only short time intervals and generally do not support very high levels of average fault current in a circuit. Nevertheless, arcing faults can produce sparks which measure 10,000 degrees centigrade or more. Thus, although arcing faults may not support readily detectable fault currents, arcing faults can produce sparks that are capable of starting many fires.
Like bolted faults, arcing faults may occur either between two wires or between a wire and ground. However, unlike bolted faults, an air gap separates the two wires or the wire and ground between which arcing fault currents must pass. Accordingly, a substantial voltage across the gap is required to overcome resistance of the gap and to permit current to pass through the gap. Current flow across arcing faults is relatively short lived because the voltage of conventional alternating current circuits (AC circuits) periodically reverses polarity and drops below a value needed to sustain current flow through the fault gap. Also, the current lags behind the voltage in phase angle and takes time to rise to a maximum value through the gap. Together, these two characteristics of arcing faults significantly limit the average amount of current that can flow through an arcing fault gap. Such low levels of average current may not be easily distinguished from other amounts of current that ordinarily flow through the circuit.
One previous attempt to detect lower levels of fault current that might be associated with arcing faults is disclosed in U.S. Pat. No. 3,911,323 to Wilson et al. A transistor amplifier is used to make the conventional detection equipment more sensitive to ground fault currents. Initially, only a warning signal is given in response to the detection of possible fault currents that are not of a sufficient magnitude to trip a circuit breaker. However, if the fault current persists and increases to a substantial level, a ground fault indicator is used to trip a circuit breaker.
Although the known fault detector may indicate the presence of arcing faults to ground, other types of arcing faults (e.g., wire-to-wire faults) go undetected. Further, the detector of Wilson et al. permits arcing faults to continue unabated for a considerable period of time. A fire may be started by the fault well before the fault current raises to the substantial level at which the circuit is interrupted.
Nevertheless, it would not be a good idea to modify the detector of Wilson et al. to interrupt the circuit in response to a possible ground fault any sooner. Other ordinary circuit disturbances can produce erroneous indications of a temporary ground fault. Moreover, it is a very serious matter to cut off the power to a circuit, and such a step should not be taken unless there is a clear presence of a fault. In a household circuit, the interruption of power shuts off the supply of heat, refrigeration, and lights; any one of which can have serious consequences. Valuable data in active computer memory can also be lost by an unexpected power loss at businesses. Further, in an industrial plant, a sudden power loss can damage equipment or parts in manufacture, and can pose a significant safety hazard.
In addition to supporting fault current, arcing faults also exhibit a variety of other electrical characteristics. However, many of the same or similar characteristics can be exhibited by other circuit disturbances that do not warrant cutting off the supply of power to the circuit. Accordingly, it has been a general practice to merely activate some form of alarm in the presence of faults that involve only small leakages of current rather than to immediately cut off the supply of power. As a result, most households, businesses, and industrial plants are not very well protected against arcing faults and are subject to electrical fires that are started before conventional fault detection equipment cuts off the supply of electrical current to the fault.